A spherical capacitor consists of two concentric, thin conducting spherical shells of radii \(a\) and \(b\), where \(a < b\). The space between the shells is completely filled with a homogeneous, weakly conducting material having a dielectric constant \(\kappa\) and a uniform resistivity \(\rho\). The capacitor is initially charged so that the inner shell carries charge \(+Q_0\) and the outer shell carries charge \(-Q_0\), and it is then isolated. Which of the following expressions correctly represents the characteristic time constant \(\tau\) for the exponential decay of charge on the capacitor?